1. Statement of the Technical Field
The present invention is directed to the field of communications. In particular, the present invention is directed to systems and methods for improving signal reception in chaotic communications systems.
2. Description of the Related Art
There are many types of communications systems known in the art, such as multiple access communications systems, low probability of intercept/low probability of detection (LPI/LPD) communications systems and spread spectrum communications systems. Many of these systems depend on spreading sequences. Other systems induce exploitable correlations via square-pulse, square pulse with pulse shaping or use frequency hopped carriers. Non-square-pulse spreading sequences have also been employed but require significantly more computational power to synchronize. Communication signals employing non-square pulse spreading sequences are typically more secure and robust against interferers.
Although spread spectrum communications provide one way of exchanging communications signals robustly or securely, such systems are still susceptible to self-interference caused by multipath images that occur in the physical transmission channel. That is, due to reflections from objects in the transmission channel between a transmitter and a receiver, many copies of the originally transmitted signal may be received at the receiver. Typically, these additional images are time-delayed and can have a different amplitude and phase as compared to the originally transmitted signal, making difficult the recognition of the originally transmitted symbols from the signal received by the receiver. When these multipath images achieve destructive interference, the received signal is said to undergo fading; the wideband nature of a spread spectrum communications signal potentially creates fading that is either flat or frequency selective. Fading effects are well understood.
One proposed method of dealing with such issues has been the use of RAKE receivers. RAKE receivers use multiple receiving elements (RAKE fingers) to receive the multiple copies of signals and can perform demodulation on selected paths and coherently combine the multiple demodulated signals, providing multipath mitigation and improved signal-to-noise (SNR) ratios. When the spreading sequence has acceptable short term and long term correlation properties, the coherent combining can be performed during the dispreading process instead of post demodulation. Direct sequence spread spectrum signals in particular typically rely on correlation-based receivers, resulting in multipath performance that is measurable on the order of a spreading chip duration; square pulse spreading chips as used in direct sequence spread spectrum systems and its multiple access extension CDMA communication systems have disadvantages due to a signal timing ambiguity within the chip, and poor short time correlation properties. More continuous amplitude spreading sequences such as chaotic or CAZAC sequences provide the capability to perform limited multipath image separation with higher resolution. Given the separation capability of these substantially more continuous amplitude spreading signals, it is possible to implement a physically realizable RAKE receiver during the dispreading process that obtain more useable SNR improvements and reduce fading effects than possible with traditional direct sequence spread spectrum systems.